This invention relates to a nitrile group-containing highly saturated copolymer rubber used as material for a crosslinked rubber product having excellent oil resistance, a crosslinkable rubber composition comprising the nitrile group-containing highly saturated copolymer rubber, and a crosslinked product of the crosslinkable rubber composition.
Longer service quality and less maintenance are required for automobiles, and thus, greater durability is required for automobile parts. Especially, less volume change with time is required for rubber parts placed in contact with oil such as gasoline and a lubricating oil.
As rubber exhibiting less volume change with time when it is placed in contact with oils, a nitrile group-containing highly saturated copolymer rubber having a reduced content of carbon-carbon double bonds, namely, a hydrogenation product of a nitrile group-containing hydrocarbon rubber ouch as an acrylonitrile-butadiene copolymer rubber, was proposed, for example, in Japanese Unexamined Patent Publication No. 854-132647. A crosslinked product of the nitrile group-containing highly saturated copolymer rubber exhibits excellent resistance to rancid gasoline and to ozone, and thus, is widely used as sealing materials and hoses in automobiles. However, the nitrile group-containing highly saturated copolymer rubber made by hydrogenating the copolymer tends to harden when it is placed in contact with oils such as gasoline and a lubricating oil, and thus, the rubber elasticity does not last for a long time. It is not known that a certain hydrogenation rubber product of a nitrile group-containing highly saturated terpolymer rubber having no tendency of hardening even when it is placed in contact with oils.
Oils such as gasoline and a lubricating oil occasionally contain trace amounts of condensed aromatics having a high boiling point such as xcex1-naphthol, anthracene and phenanthrene, which are residual ingredients remaining without separation in a step of refining crude oil. It is presumed that the hardening in oil of a nitrile group-containing highly saturated copolymer rubber occurs due to the fact that the condensed aromatics hinder movement of a high polymer chain in rubber. However, it is difficult to remove the trace amounts of condensed aromatics in a step of refining crude oil and to produce refined oil products such as gasoline and a lubricating oil, which do not harden the nitrile group-containing highly saturated copolymer rubber. Therefore, there has been a demand for an improved nitrile group-containing highly saturated copolymer rubber exhibiting reduced tendency of hardening even when it is placed in contact with refined oil products.
A highly saturated copolymer rubber having an improved cold resistance is known, for example, in Japanese Unexamined Patent Publication No. S63-95242, which is made by hydrogenating a copolymer prepared by copolymerization of an xcex1,xcex2-ethylenically unsaturated nitrile monomer, a conjugated diene monomer and an unsaturated carboxylic acid monomer. A crosslinked product of this highly saturated copolymer rubber contains only a minor amount of nitrile monomer units, and thus, the rubber tends to exhibits a large change in volume when it is placed in contact with oil, especially gasoline.
A primary object of the present invention is to provide a nitrile group-containing highly saturated copolymer rubber which is used as rubber material giving a crosslinked rubber product exhibiting a reduced tendency of hardening and being changed in volume even when it is placed in contact with oils containing residual condensed aromatics, while good resistance to rancid gasoline is maintained, and further to provide a crosslinkable rubber composition comprising the nitrile group-containing highly saturated copolymer rubber, and a crosslinked rubber product made from the crosslinkable rubber product.
To achieve the above-mentioned object, the present inventors made extensive research and found that a crosslinked rubber product made by crosslinking a nitrile group-containing highly saturated copolymer having a special monomer unit composition exhibits greatly reduced swelling in oil, excellent resistance to rancid gasoline and greatly reduced hardening in oil. Based on this finding, the present invention has been completed.
Thus, in one aspect of the present invention, there is provided a nitrile group-containing highly saturated copolymer rubber comprising (a) 0 to 20% by weight of 1,3-butadiene units, (b) 0 to 50% by weight of saturated 1,3-butadiene units, (c) 40 to 50% by weight of xcex1,xcex2-ethylenically unsaturated nitrile monomer units, and (d) 10 to 35% by weight and at least 8% by mol of other monomer units, wherein the sum of 1,3-butadiene units (a) and saturated 1,3-butadiene units (b) is in the range of 20 to 50% by weight.
In another aspect of the present invention, there is provided a crosslinkable rubber composition comprising the above-mentioned nitrile group-containing highly saturated copolymer rubber and a crosslinking agent.
In still another aspect of the present invention, there is provided a crosslinked rubber product made by crosslinking the above-mentioned crosslinkable rubber composition.